An edge-light type surface light source device in which a light guide plate is employed has come into widespread use mainly as a backlight for a liquid crystal.
An edge-light type surface light source device has become prevalent in a backlight for a liquid crystal. This is because the edge-light type surface light source device is more effective in thinning (i) a backlight module for a liquid crystal and (ii) a product to which the backlight module is applied, as compared with a direct type backlight. Note that (a), in the direct type backlight, a light source is arranged directly under a liquid crystal panel instead of using a light guide plate, whereas (b), in the edge-light type surface light source device, (i) a linear light source is arranged on an edge of a light guide plate and (ii) the light guide plate converts light into plane emitting light. Furthermore, in some cases, the edge-light type surface light source device is employed also as an illumination.
Conventionally, a cold cathode fluorescent lamp (CCFL) has been prevalent in a light emitting source of such a light source device. Note, however, that the cold cathode fluorescent lamp (CCFL) has been recently replaced with a light emitting diode (LED). This makes it possible to (i) eliminate use of mercury, which (a) is employed in the CCFL and a fluorescent tube and (b) imposes a heavy burden on the environment, (ii) reduce power consumption, (iii) increase color reproducibility, and (iv) prolong a life of the light source device.
In such a structure, there may be a difference in luminance between a part thereof that is weakly irradiated with light from a light source and the rest thereof. Given these circumstances, backlight units have been proposed which make it easy to reduce a difference in luminance of a liquid crystal panel (see Patent Literatures 1 to 3).